A car battery may be inadvertently connected to the car wiring the wrong way round, i.e. with the wrong polarity. For example, during a jumpstart procedure, a second, external battery is connected to the terminals of the main battery, so as to provide a supporting voltage source during the cranking period. Accidental connection to the car wiring in reverse polarity may occur. As electronic devices are becoming important as integral parts of vehicles in greater quantities, there is a need for special provisions in each electronic device to avoid damage to circuitry and components as a result of inadvertent reverse voltage at the power supply.
It is common practice, when dealing with low or medium currents and the power loss and heat generation is low, to provide protection by inserting a serial diode between the power supply and component/circuit. However, these simple diodes cause undesirable power loss during normal operation of the devices; the power loss being equal to the forward voltage drop of the corresponding diode multiplied by the average current flowing.
Power semiconductors (even in power distribution boxes) are being used more and more nowadays as substitutes for mechanical relays and also the provide circuit protection (fusing) functionality. It is common to use power field effect transistors (FETs) in such devices as they provide a very low ON-Resistance down to 0.5 to 1 ohm and are able to carry a huge amount of permanent drain current in the range of 50 A to 100 A. A typical power FET becomes conductive via its intrinsic diode when connected in reverse polarity. This has to be avoided because high current leads to a very high power loss in the reverse diode of the FET. The consequential power dissipation may be 50 or even 100 W and would destroy the FET by overheating in a very short time leading to a great risk of fire hazard.
To overcome this problem a second power FET is known to be implemented in series arranged in opposite directions (with source of the first transistor connected to the source of the second one) to each FET switch to avoid reverse current flow. This leads inevitably to doubling of the resulting ON resistance, power loss and material costs.
FIG. 1 shows a schematic representation of how components and systems of a vehicle are protected in prior art systems. The circuit 1 shows a power supply 2 i.e. a battery, which may be incorrectly connected i.e. with reverse polarity. The battery can supply power to one or more low power loads 3 or one or more high power loads 4. To protect the low power loads diodes 5 are used between the power supply and the low power loads 3 (components). To protect the high power loads 4 (components and devices) an additional FET 6 is provided to an existing FET 7 to provide a pair of FETs arranged in series. So a second power FET 6 is implemented in series but in arranged in the opposite direction to each other (with source of the first transistor connected to the source of the second one) to avoid reverse current flow. As mentioned this leads inevitably to doubling of the resulting ON resistance, power loss and material costs.
It is known to use latching relays in vehicle systems located between the battery and the fuses and loads of vehicle systems for purposes other than to prevent damage form reverse polarity connection. FIG. 2 shows a schematic circuit diagram showing such a system. The latching relay 9 is operated by a SET coil 10 (in series with resistor 11) and a RESET coil 12 (in series with resistor 13). The latching relay 9 is controlled by SET and RESET pulses 14, 15 sent to inductive load drivers 16, 17. These inductive load drivers 16, 17 control the currents sent through the SET and RESET 10, 12 coils which in turn open/close the latching relay 9.